Synthesis and luminescence properties of orange-red-emitting M₂Si₅N₈:Eu²⁺ (M = Ca, Sr, Ba) light-emitting diode conversion phosphors by a simple nitridation of MSi₂

Eu²⁺-doped M₂Si₅N₈(M = Ca, Sr, Ba) orange-red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi₂, Eu₂O₃, and α-Si₃ N₄ as the starting materials under N₂-H₂ (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M₂Si₅N₈ for M = Ca, Sr, and a little amount of BaSi₇N₁₀ impurity phase for M = Ba. Under the blue light excitation, M₂ Si₅N₈:Eu²⁺ shows a typical broad band emission of Eu²⁺ ranging from orange to red (585-620nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca<Ba<Sr, which suggests that Sr₂Si₅ N₈:Eu²⁺ has the highest application potential as a red conversion phosphor for white light-emitting diodes.